It has become common place to route optical signals within optoelectronic devices using waveguides created within a medium housing other circuit components. These devices are known as Photonic Integrated Circuits (PICs) and the passive waveguides are used to route optical signals between active devices on such circuits. Waveguides are also used in such circuits to route optical signals to/from other circuits, usually via fiber ports.
As the density of such PICs increases, so does the need for sharp turns along the path of the waveguides. These sharp turns are often on the order of 90 degrees. To avoid radiation loss of the optical signal at the bends, waveguides with high index contrast medium are used. The high index waveguide tightly confines the optical modes in the lateral direction by the large index discontinuity between the waveguide core and the surrounding medium (e.g., dielectric/air). However, because of the tight optical confinement such high index contrast waveguides experience more scattering loss from roughness in the waveguide side-walls (introduced during manufacturing) than do low index contrast waveguides.
Waveguides with low index contrast (e.g. ridge-loaded waveguide, buried waveguide, etc.) are preferable for single mode operation due to their larger cross-sectional dimensions (>1 μm) allowing for relatively easy fabrication using conventional optical lithography and the fact that they are better mode-matched to optical fiber which results in low power coupling loss. However, low index waveguides require a relatively large (greater than a few mm) bending radius to avoid excessive attenuation due to radiation loss. Thus, low index waveguides are not suitable for use in devices requiring sharp bends along the optical signal path.
FIGS. 1A and 1B show prior art waveguide 10 having discontinuity interface 11. As shown on FIG. 1A, light enters the waveguide at input port 12 and progresses within the waveguide as an optical mode until it reaches discontinuity 11 which, in the embodiment, is an air interface. Edge 19 of discontinuity 11 is approximately 45° to the direction of travel of the light. As will be seen from FIGS. 4A and 4B, because of the 45° angle and the difference in index of refraction between the medium (waveguide core) of waveguide 10 and the medium of discontinuity 11 (air), the optical mode experiences TIR and is channeled within the waveguide to output 13. In this manner, a sharp bend (in the order of 90°) is effectuated by the air interface of TIR turning mirror 11. The turning angle can be greater than 90°, as long as the incident angle of the input beam on the TIR mirror is greater than the critical angle.
The waveguide shown in FIG. 1A illustrates a ridge waveguide with ridge 14 above upper cladding 15 which in turn is above waveguide core 16. Below waveguide core 16 is lower cladding 17, all of which is constructed above substrate 18. Note that the entire structure 10 is typically constructed as a unitary structure by etching away certain portions, all as is well-known. The air around the ridge forms an index step to confine the wave laterally. Ridge 14 and the upper cladding can be the same material or different material. Waveguide core 16 is a continuous slab, except for air trench 11 which forms the TIR mirror which is etched through the waveguide core and into lower claddings layer 17. Guided optical waves that are to pass through waveguide 10 enter input port 12 and exit output port 13. Outline 101 shows a typical guide mode profile.
FIG. 1B is a view of waveguide 10 where ridge 14 has been lifted for clarity. Trench 11 is created by anisotropically etching down to the lower cladding of the waveguide to avoid radiation loss to the lower cladding and substrate at the TIR mirror. Ideally all of the energy of the guided wave should be reflected by the dielectric air interface and should enter output wave portion 13. However, as shown by guide mode profile 101 (FIG. 1A), a significant amount of energy of the guided wave extends ridge width “w” due to weak lateral index guiding.
The energy tails of the lateral mode profile experience additional phase delays when the guided wave traverses the waveguide core region being reflected from front surface 19 (FIGS. 1A and 1B) of TIR discontinuity 11. The extra phase delay distorts the guided wave front and results in diffraction loss.
FIG. 4 illustrates one prior art method of constructing discontinuity interface between guide portions 12 and 13. TIR mirror 43 is constructed with two separate mask levels. The first mask level is used to define waveguide portions 12 and 13, and the TIR mirror is defined by both mask levels 1 and 2. The process flow is shown in FIGS. 6A-6D and 7A-7D and illustrates why misalignment changes the TIR mirror interface. Even with submicron alignment accuracy tools, there is no guarantee that the shape and position of the turning mirror can be reproduced within a given accuracy range. Slight misalignment will have large effects on the efficiency of the turning mirror to perform its intended function.
FIG. 1C shows a cross-section view of prior art waveguide 10 (as shown in FIG. 1A) taken along line IC-IC and shows one prior art method of constructing the discontinuity interface.
FIG. 1D shows a cross-section view of prior art waveguide 10 (as shown in FIG. 1A) taken along line 1D-1D